DESCRIPTION (ADPATED FROM APPLICANT'S ABSTRACT): Mycoplasmas are, widely distributed in nature and commonly produce disease in plants, insects, and animals, including man. Although the economic impact of these diseases is great, little information is available concerning mechanisms of pathogenesis and effective methods of control are unavailable. The surface properties of numerous mycoplasma species vary at high rate, about 10-3 per cell per generation. In the murine pathogen Mycoplasma pulmonis, high-frequency phenotypic variations involving changes in variable surface antigens (VI antigens) affect colony morphology, the susceptibility of the organism to mycoplasma viruses, and the adsorption of mycoplasmas to red blood cells. Most of the high-frequency phenotypic variations in M. pulmonis result from site-specific DNA inversions occurring in the hsd loci (encoding complex, phase-variable restriction and modification systems) and the vsa locus (encoding the phase-variable V-1 surface proteins). The finding of phase-variable restriction and modification systems is novel and challenges the notion that the function of restriction systems in bacteria is limited to the protection of cells from invasion by foreign DNA (phage infection). This challenge takes on added dimension by the observation that DNA inversions within vsa and within hsd apparently occur in concert. The long-range goals are to understand the molecular basis of mycoplasmal chromosomal and phenotypic variation and the role these variations have in disease pathogenesis. Completion of these goals is essential for the development of control measures for mycoplasmal diseases and into pathogenic mechanisms of bacterial diseases. The genome of M. pulmonis will have been fully sequenced by the end of 1999. This application proposes to capitalize on the forthcoming sequence information through the construction of a transposon library of M. pulmonis mutants. In aim 1, the chromosomal site of transposon insertion will be determined for each mutant to precisely identify the mutated gene. In aim 2, mutants with defects in putative DNA recombinases and V-I protein production will be examined to study the mechanisms of gene rearrangements and phenotypic switching.